As is wont with modern open source software projects, the librarian-puppet homepage describes itself in terms that only fully make sense after you already know what the software does:
Librarian-puppet is a bundler for your puppet infrastructure. You can use librarian-puppet to manage the puppet modules your infrastructure depends on, whether the modules come from the Puppet Forge, Git repositories or a just a path.
The description I wish I could have read as an introduction might have been less accurate but more concrete, like:
librarian-puppet command line program reads a list of puppet modules from a local text file
Puppetfile and downloads those modules onto your disk. You can specify specific versions of the modules you want to download and, as you make changes to the
librarian-puppet will download new module versions or delete modules from the disk to match the